Oven stripper



Aug. 28, 1962 R. F. MERTZ 3,051,296

OVEN STRIPPER Filed Nov. 27, 1959 4 Sheets-Sheet 1 INVENTOR.

RALPH F. MERTZ ATTO R S Aug. 28, 1962 R. F. MERTZ 3,051,296

OVEN STRIPPER Filed Nov. 27, 1959 4 Sheets-Sheet 2 INVENTOR.

RALPH F. MERTZ o,- NEYS R. F. MERTZ OVEN STRIPPER Aug. 28, 1962 4SheetsSheet 3 Filed Nov. 27, 1959 .INVENTOR.

RALPH F. MERTZ ,fldodaf Qyw ATTOR YS Aug. 28, 1962 R. F. MERTZ 3,051,296

OVEN STRIPPER Filed Nov. 27, 1959 4 Sheets-Sheet 4 INVENTOR.

RALPH F. MERTZ 36w M/Qww ATTO NEYS ilnited tates Fatent @iiim 335L295Patented Aug. 28, 1952 3,051,296 OVEN STREPER Ralph F. Mertz, ValleyCottage, N.Y., nssignor to Miehle- Goss-Dexter, Incorporated, (Zhicago,Ill., a corporation of Delaware Filed Nov. 27, 1959, Ser. No. 855,854 11Claims. (Cl. 198134) This invention relates to sheet handling apparatusand more particularly to apparatus for receiving sheets of material fromthe conveyor means of a sheet treating apparatus, such as a drying oven,and moving the same out of operational contact with said conveyor means.

In the sheet metal art it has been found desirable to treat metal sheetsin various ways such as coating said sheets with a protective and/ ordecorative material. The sheets of metal carrying the protective coatingare then transported through a drying oven on a conveyor means. Thisinvention is directed to apparatus for removing the sheets from the ovenconveying means without stopping said conveying means.

One object of the present invention is to provide an oven strippingapparatus which sequentially receives a series of sheets of material andquickly moves each sheet out of operational range of the means forconveying the sheets through the oven.

Another object of the present invention is to provide an oven strippingapparatus wherein the sheets are firmly seated and retained on aconveyor means with a minimum of sheet surface contact with the conveyormeans.

Another object is to provide means associated with a conveyor forattracting sheets toward the conveyor and to firmly retain said sheetson the conveyor to prevent relative motion between the surface of thesheets and the conveyor whereby marring of the sheet surfaces isprevented.

Another object is to provide a drive means for a conveyor having meansfor adjusting the speed of the con veyor during the operation of theconveyor.

Another object is to provide an oven stripping apparatus having meansfor preventing the stopping of the oven conveyor means when a jam occursbetween said oven conveyor means and a sheet seated on said stripperconveyor.

Another object is to provide novel means for supporting an ovenstripping apparatus.

A still further object is to provide an oven stripping apparatus havingnovel support means therefor and novel means responsive to a jam betweena sheet on the stripper conveyor and the oven conveyor to cause saidsupport means to be actuated whereby said stripper conveyor and sheetare moved away from said oven conveyor.

The above and further objects and novel features of the invention willmore fully appear from the following detailed description when the sameis read in connection with the accompanying drawings. It is to beexpressly understood, however, that the drawings are for the purpose ofillustration only and are not intended as a definition of the limits ofthe invention.

In the drawings, wherein like reference characters refer to like partsthroughout the several views,

FIG. 1 is a top plan View of the oven stripper;

FIG. 2 is a side elevational view taken along line 22 of FIG. 1;

FIG. 3 is a side cross-sectional view of the suction means taken alongline 33 of FIG. 1;

FIG. 4 is an end crosssectional view of the suction means taken alongline 4-4- of FIG. 3;

PEG. 5 is a cross-sectional view of the supplemental magnetic hold downmeans; and

FIG. 6 is an end elevational view of the oven stripper taken along line6-6 of FIG. 1.

Referring to the drawings, one form of oven stripping apparatusconstructed and operated in accordance with the present invention isherein illustrated as being operatively associated with a drying ovenconveyor system. It is to be understood that this apparatus couldequally be associated with other types of sheet treating apparatuseswherein it is desired to quickly and efliciently remove sheets from acontinuously moving conveyor system. In an apparatus for removing sheetsof metal having coated or printed surfaces from the continuouslyoperating oven conveyor, it is necessary to provide means to receiveeach sheet and quickly move the same away from the oven conveyor withoutpermitting the surfaces of the sheets to be marred and also to releaseany jams which might occur between the oven conveyor and a sheet on theoven stripper. To prevent any slippage between a sheet and the stripperconveyor there is illustrated a suction means to hold the sheets on theconveyor. The suction comprises a low pressure differential on each sideof the sheet with the suction applied over the large area of the sheetto provide a downward force of sufficient intensity to retain the sheetson the conveyor without slippage. At the same time this downward forceis not great enough to prevent the sheet from being transferred from thestripper conveyor to another conveyor.

Referring to FIGS. 1 and 2, an oven stripper 15 is mounted to one end ofan oven 14 and retained in alignment with an oven conveying means.Sheets of metal are conveyed through a conventional drying oven (notshown) in a vertical position and are supported by spaced apart wicketmembers it which are in turn secured at each side to a pair of endlessconveyor chains 11. The oven conveyor chains t1 are continuouslyoperated at a uniform speed. At the discharge end of the oven theconveyor chains are driven over three pairs of sprocket members arrangedin a generally triangular position to each other as shown in H6. 2 sothat each wicket and sheet 12 supported by it are first carried at agradual downward slope toward a horizontal plane and then each wicketabruptly reverses its direction as it moves away from the horizontalplane and continues to the lower path of travel of the oven conveyorchains 11 to be returned to the position for receiving additional sheetsfor conveying through the oven. The oven stripper comprises a pair ofspaced apart side arms 13 having the rear portion of each pivotallymounted to the discharge end of the oven frame. As shown in FIG. 2, theside arms 13 are maintained in a horizontal plane slightly below thecenter line of the pair of sprockets around which each of the wicketscarried by the oven conveyor chains reverses its stripper mechanism andthen the wicket passes the stripper at an increased radial speed leavingthe sheet resting on the stripper. At the same time this increases thedistance between wickets at this point which allows time for the sheetto be removed before the next wicket passes the stripper.

A shaft 16 extends transversely of the forward end of the side arms 13and is rotatably mounted in suitable hearings in said side arms.Conveyor belts 17, 18 and 19 are driven from pulleys secured to shaft16. A pair of downwardly extending support plates 20 are secured to theforward ends of side arms 13. A motor adjustment shaft 21 extendsthrough the lower portion of the support plates 20 and is rotatablysupported therein by suitable bearings. An electric motor 22 is securedto the one end of the shaft 21 by a pair of brackets 23, as shown inFIG. 1. A plate 28 having an aperture for receiving an end of shaft 16extends between that shaft and the motor adjustment shaft 21. An armbracket 29 is secured at one end to the motor shaft 21 and is bifurcatedat the upper end with a block member having a threaded aperturetherethrough for receiving a threaded portion of rod 30. A rod endbearing 31 is securely attached to the plate 28, and permits rod 30 toalign itself with the threaded aperture in the block retained by arm 29.A wheel is secured to the upper end of rod 30 to facilitate turning thesame. As the rod 30 is manually rotated the arm 29 is pivotally movedcausing the shaft 21 to rotate. As shaft 21 rotates it carries with itthe motor brackets 23 and the motor 22. A drive pulley 3 2 is secured tothe motor shaft and a drive pulley 33 is secured to the conveyor driveshaft 16 with a V-belt 34 connecting said pulleys. The drive pulley 32is of the type wherein one half is secured to the motor shaft and theother half is spring-loaded along the plane of the motor shaft andtoward the fixed half of the pulley. The motor 22 and its mounting arerotated with shaft 21 which, in turn, is spaced from but in verticalalignment with shaft 16. The rotation of shaft 21 will vary the distancebetween the center of pulley 32 and pulley 33 so that this distance iseither increased or decreased depending upon the direcion of therotation of the shaft 21. Since the V-belt 34 is of a fixed length thelengthening or shortening of the distance between the center of thesetwo pulleys will cause the V-belt to ride up and down in spring-loadedpulley 32 thus changing the pitch ratios. In this manner the speed ofthe conveyors 17, 18, '19, 39 and 40 can be selectively controlled whilethe sheet is in operation. The rear portions of conveyor belts 17 and 19are supported on pulleys which are secured to shafts 35 and 36 which, inturn, are supported in shaft blocks secured to the inside of side arms13. Shafts 37 and 38 are posi tioned parallel to shafts 35 and 36 andlikewise are retained in shaft blocks which are secured to the innersurfaces of the side arms 13. A pair of conveyor belts 39 are mounted onsuitable pulleys which are secured to shafts 35 and 37, respectively,and a pair of conveyor belts 40 are mounted to suitable pulleys whichare secured to the ends of shafts 36 and 38. Each of shafts 3538 areadditionally rotatably supported by a pair of brackets 45 and 46 each ofwhich is positioned parallel to one of the side arms 13 and secured atone end to the torque tube 44. As shown in FIG. 1, conveyor belts 39 and40 are of a lesser length than conveyor belts 17 and 19. The pair ofconveyor belts 18 are mounted intermediate the pairs of conveyor belts39 and 40. The rearward pulleys for the conveyor belts 18 are rotatablymounted to a shaft which extends between a pair of upwardly extendingshaft support plates 41. The bottom portion of each plate 41 is securedto a horizontally extending bracket 42, Bracket 42 is maintained in ahorizontal position by having its forward end secured to the torque tube44 which extends transversely of the side arms 13 and is mounted in thesupport plates 20. Tube 44 is positioned in vertical alignment with andbetween the conveyor shaft 16 and the motor adjustment shaft 21.

A suction device 43 is positioned between each of the pairs of conveyorbelts 18, '39 and 40. Each of the suction devices 43 are generallyidentical in construction and operation and therefore a description ofone is sufficient for the understanding of all. As shown in FIGS. 3 and4, each suction device comprises a chamber 49 positioned below theconveyor belts, an outlet 50 at the bottom of the chamber, and a hose 56which is connected to the suction side of a blower (not shown). A pairof spaced apart wall members 51 and connecting end walls 53 form anarrow rectangular shaped passage 52 communicating with the chamber 49at its lower end and with the atmosphere at its upper end. Secured tothe upper end of each wall 51 and extending horizontally outwardly awayfrom the passage 52 are a pair of flanges 54 which are positioned incontact with the undersurface of the upper reaches of the conveyorbelts. It will be seen from FIG. 1 that the shape of the wickets willcontrol the positioning of the conveyors 17, 18, 19, 39 and 40 so thateach of the wickets, carried in their predetermined path by the conveyorchains 11, will pass these conveyors Without contact and at the sametime will leave each sheet 12 retained on the top surface of theseconveyor belts. When a suction is created in each of the suction devices43, air will be drawn into the passages 52, thus creating an increasedflow of air above the conveyor belts toward these suction devicesbetween the belts. This, in turn, creates a downward movement of airtoward the conveyor belts on the underside of each sheet as it advancestoward said conveyor belts and results in an additional downward pull oneach advancing sheet toward the conveyor belts. The upper surface ofeach of the conveyor belts is composed of a series of paralleltranver-sely extending grooved members. When each sheet has been seatedon the conveyor belts the flow of air is restricted to these groovedareas as well as the narrow area defined by the undersurface of thesheet, the upper surface of the flanges 54 and the inside edge portionsof the upper reaches of the conveyor belts. The grooved areas in theconveyor belts distribute the hold down force over a larger sheetsurface area and the restricted flow of air increases the suction on thelower surface of each sheet which, in turn, more firmly secures thesheet to the conveyor belts. In addition to the more even distributionof the suction force on a large surface area, the grooved belts providefor a minimum of contact of the sheet surface with the conveyor beltswhich, in turn, minimize the potential damage to the sheet surface. Theconveyor belts are continuously driven and quickly move each of thesheets away from the oven and onto the continuously driven endlesstake-off conveyor belts 55 positioned adjacent the forward end of ovenstripper. In this manner each sheet is quickly removed from the path ofthe wickets '10 and the stripping conveyors are cleared ready to receivethe succeeding sheet 1 2. It will aso be observed that the sheets arenot drawn tightly down into contact with the open end of the passages52. By having the upper end of passage 52 ending at a horizontal planewhich also passes through the undersurfaces of the conveyor belts, thereis provided a passageway equal to the length of the slot 52 and having awidth equal to the spacing between the conveyor belts and a depth equalto the thickness of the conveyor belts through which air is constantlydrawn. This permits a partial suction on the undersurface of each sheetcarried on the conveyor to create a holding force but at the same timethe force is not great enough to impede the efficient transfer of eachsheet onto the take-off conveyor belts 55 nor to damage the sheets.Should the undersurface of the sheets 12 be permitted to seal off theupper end of passages 5-2, the suction would have to be cut off torelease the sheet to permit its transfer onto the take-off conveyorbelts 55. A frame comprising vertically-extending supports 60 andhorizontally-extending side frames 61 is positioned forward of the ovenstripper 15. The supports 60 are secured at their lower ends to a baseor floor (not shown) and have a conveyor belt shaft 62 extending betweentheir upper ends and rotatably mounted therein by suitable. bearings.Conveyor belts 55 are operatively connected at one end to pulleys whichare secured to shaft 62 and at their other end to pulleys and a driveshaft (not shown) for continuous operation.

The forward end of the oven stripper 15 is pivotally retained in ahorizontal plane by a stripper holding means generally designated as 65and more specifically described as follows. Secured to frame 60 is anair cylinder 66 of standard construct-ion having a reciprocating pistonand a piston rod 67 which extend exteriorly of one end of the casing ofthe cylinder. Aconduit 68 is secured at one end to a source ofcompressed air (not shown) and at the other end to a valve 69 which isof the commercial type made by the Bellows Co., Model No. 4SS-V3/8 andwhich is electrically actuated to selectively direct the compressed airto either end of the air cylinder, which, in turn, will move the pistonto one end or the other of the cylinder. A shaft 72 is rotatably mountedbetween the upper ends of the support 60 by suitable bearings and has asprocket 73 secured to one end thereof. A chain is secured at one end tothe sprocket 73 and at its other end to the outer end of piston rod 67.Secured to the forward surface of the torque tube 44 are a pair ofcontrol devices 74 and 75. Since each control device is of identicalconstruction and operation, a description of one will be sufficient foran understanding of both. Secured to the torque tube 44 is a base plate76. A lever arm 77 is pivotally monted at 78 to the base plate 76 andcompresses a short arm 79 and a long arm 80. A pin 81 is secured at itsupper end to the bracket 76 and extends downwardly through an aperturein the end of arm 80. A spring 82 is telescopically mounted over the endof pin 81 and is retained under slight compression between theundersurface of arm 80 and a nut which is threadedly received on the endof pin 8. A microswitch 83 having an actuating pin 84 is mounted toplate 76 so that the pin 84, in its normal nonactivated position,contacts the upper surface of arm 89.

The microswitch is of the type which has a springloaded pin 84 and theswitch can be wired either in the normally opened or normally closedposition when the pin is recessed into the casing, as shown in FIG. 6.The microswitch of the control 74 is wired so that the switch isnormally opened when the spring-loaded pin 34 is recessed into thecasing. The microswitch of control device 75 is wired so that the switchis normally closed when the spring-loaded pin 84 is recessed into thecasing. Spring 81 holds arm 89 in an upward position and arm 81, inturn, retains the spring-loaded pin 34 in its recessed position. A chain88 is secured at one end to the outer end of arm 79 and has its otherend secured to a sprocket 89 in such a manner that a portion of thechain engages the teeth in this sprocket. As shown in FIGS. 1 and 6,there is a pair of sprockets 89 and chains 88, each of which isoperatively connected to the control devices 74 and 75, respectively.The spring member 82 associated with control device 75 is larger thanthe spring associated with control device 74 in order to compensate forthe added weight of the motor and the drive pulleys attached to thatside of the oven stripper. The microswitch associated with the controldevice 74 is electrically connected to the Bellows valve 69. Themicroswitch associated with the control device 75 is electricallyconnected to the control circuit for the drive of the oven conveyorchain 11 (not shown).

In normal operation the oven stripper will remain in a horizontalposition as shown in FIG. 2 and the sheets will be conveyed onto thetake-ofi conveyor belts 55 by the conveyor belts which form a part ofthe oven stripper. The forward end of the oven stripper is supported bythe pairs of chains 88 and 74 and the air cylinder 66. Any downwardpivotal movement of the oven stripper is resisted by the compressed airon the topside of the piston in air cylinder 66. Should a sheet bejammed between the conveyor belts of the oven stripper and the nextadvancing wicket It the entire oven stripper is slightly pivotally moveddownwardly. When this occurs, the arm will be pivotally moved downwardlyand the arm 79 will be moved upwardly which, in turn, will permit thespring-loaded pin 84 to be moved downwardly thus closing the microswitch83 in each of the control devices 74 and 75. The closing of themicroswitch in the control device 74 will cause the Bellows valve 69 todirect the compressed air to the underside of the piston in the aircylinder 66 thus moving the piston rod 67 upwardly (as viewed in FIG. 2)which, in turn, permits the sprockets 73 and 89 to be rotated in acounterclockwise direction. The forward end of the oven stripper is thuspermitted to drop rapidly to relieve the tension between the wicket andthe sheet. At the same time the microswitch of the control device 75 hasbeen closed, which, in turn, opens the circuit for driving the wickets10. As soon as the jammed sheet has been moved onto the take-offconveyor belts 55, the springs 81 will return the lever arms to theirnormal position thus opening the microswitch 83 and the cycle isreversed, wherein the Bellows valve 69 is deactivated, compressed air isdirected to the top side of the cylinder of the pistion in the aircylinder 66, which, in turn, pulls the chains 74 downwardly and thechain 38 upwardly and returns the oven stripper to its normal horizontalposition. It will be observed from FIG. 2 that the conveyor belts of theoven stripper 15 are normally maintained at a slightly higher elevationthan the take-off conveyor belts 55. This arrangement is designed topermit a sheet on the conveyor belts of the oven stripper to be feddirectly onto the conveyor belts 55 when the oven stripper and itsconveyor belts are pivotally depressed downwardly, as described above,at the time that a jam occurs.

A distributing manifold 92 is mounted to the supports 60. A source ofsuction (not shown) is connected through conduit 93 to the manifold 92.A hose 56 connects the manifold to each of the suction devices 43 andthe intensity of the suction is selectively controlled by varying thesize of the hose 56.

On each side of the stripper there is a vertically-extending and freelyrotatable guide roll 96 which is mounted to a horizontally extending rod97 which, in turn, is mounted to the inner surface of the side rail 13.The rolls 96 are adjustably secured to shaft 97 so that they may bemoved toward or away from the side rails to accommodate sheets ofvarious widths.

A pair of support rollers 97 are mounted to shaft 16 and are so spacedthereon to support each sheet between the conveyors. When the sheets areformed of magnetizable material the rollers 97 may be in the form ofmagnets. As shown in FIG. 5 the core 93 surrounds shaft 16 which, inturn, magnetizes the end plates 99 and 1%. In the embodiment shown,plate 99 is the north pole and plate 10!} is the south pole with themagnetic [field extending generally horizontally as shown by the arrows.The magnets not only serve to support each sheet intermediate theconveyors, but the magnetic field serves to attract and retain ametallic sheet onto the conveyors to further prevent slippage.

While the foregoing description and the accompanying drawings do setforth with more or less particularity one embodiment of the invention,is to be expressly understood that said invention is not limited to saidembodiment. Various changes may be made in location, size and intensityof the suction means as well as in the design and arrangement of theother parts illustrated, and in the mode of operation and manner of use,without de parting from the spirit and scope of the invention, as willnow be clear to those skilled in the art.

What is claimed is:

1. In an apparatus for receiving and conveying sheets of material thecombination of a plurality of spaced apart conveyor means, means tosequentially feed sheets onto said conveyor means, means for creating asuction between said conveyor means, a frame pivoted at one end forsupporting one end of said conveyor means, means operatively secured tothe other end of said frame to retain the same in a substantiallyhorizontal position, means to release said frame retaining means, andmeans responsive to a downward pressure on each conveyor in excess ofthe weight of said sheet to actuate said frame releasing means.

2. In an apparatus for receiving and conveying sheets of material, thecombination of a plurality of spaced apart conveyor means, means tosequentially feed sheets onto said conveyor means, means for creating arestricted flow of air on the undersurface of each sheet and distributedevenly about the peripheral edge of each sheet for retaining each sheeton said conveyor means, a frame pivoted at one end for supporting oneend of said conveyor, means to releasably retain said frame in asubstantially horizontal position, means to release each frame retainingmeans, and means responsive to a downward pressure on said conveyor inexcess of the weight of said sheet to actuate said frame releasingmeans.

3. Apparatus as defined by claim 2. wherein said frame retaining meanscomprises an air cylinder having a reciprocable piston and a connectingpiston rod, a source of compressed air selectively connected to eitherthe first orsecond side of said piston, chainmeans connected to saidpiston and to the forward end of said frame, control means between saidcylinder and said source of compressed air to normally direct saidcompressed air to the first side of said piston whereby the forward endof said frame is retained in a substantially horizontal position.

4. Apparatus as defined in claim 3 wherein said means responsive to anexcessive downward pressure on said conveyors actuates said controlmeans to direct compressed air to the second side ofsaid piston wherebythe forward end of said frame is permitted to rapidly drop.

5. In an apparatus for receiving and conveying sheets of material thecombination of a plurality of spaced apart conveyors, means tosequentially feed sheets onto said conveyors, means for creating -asuction between said conveyors, a frame for supporting said conveyorsand pivotally mounted on one end, means to drive said conveyors, meansto adjust the speed of said drive means while said conveyors are inoperation, means at the other end of said frame to retain the same in asubstantially horizontal position, means to disconnect said frameretaining means, and means responsive to a downward pressure on saidconveyor in excess of the weight of each sheet to actuate said framedisconnecting means.

6. Apparatus as defined in claim 5 comprising a mo tor support shaft, amotor secured to said support shaft, a spring loaded split drive meansconnected to said motor, and means to rotate said support shaft toselectively vary the distance between the centers of said conveyor drivemeans and said motor drive means.

7. A sheet handling apparatus comprising a plurality of sets of spacedapart pairs of conveyors, means for creating a suction between saidconveyors, means for sequentially depositing sheets of material on saidconveyors, said last named means mounted to pass between said pairs ofconveyors and to leave each sheet resting on said conveyors, a frame forsupporting said conveyors and pivotally mounted at one end, means at theother end of said frame to retain the same in a generally horizontalposition, means to disconnect said frame retaining means, and meansresponsive to a jam between said sheet depositing means and a sheet onsaid conveyors to actuate said frame disconnecting means.

8. In a sheet conveying apparatus the combination of a first and asecond conveyor means the respective paths of which intersect at planessubstantially normal to each other, the first conveyor comprising atleast a pair of endless belt members, means to cause a low velocity andhigh volume of air to continuously pass downwardly only through thespace between said belts whereby each sheet is retained on the topsurface of the upper reaches of said belts and wherein each sheet moveswith said belts without slippage therebetween, said last mentioned meanshaving an opening which is positioned below the top surface of the upperreaches of said belts whereby said sheets are prevented from adhering tothe said openings, the second conveyor comprising a plurality of sheetsupport means each of which is adapted to retain a sheet thereon only bygravity and each of said support means being so positioned that theypass through the plane of the belts of said first conveyor withoutcontact.

9. In a sheet conveying apparatus as defined in claim 8 wherein the topsurface of said belts on said first conveyor comprises a plurality ofparallel transverse grooves communicating the surrounding atmospherewith the opening in said means to cause a low velocity and high volumeof air to pass between said belts.

10. In a sheet conveying apparatus the combination of a first and asecond conveyor means the respective paths of which intersect at planessubstantially normal to each other, the first conveyor comprising atleast one pair of endless belt members, means to cause said belt membersto move at a predetermined surface speed, means for continuouslycreating an atmospheric pressure only between said belt members which islower than the atmospheric pressure surrounding said belt members,whereby each sheet is retained on the top surface of the upper reachesof said belts and wherein each sheet moves with said belts withoutslippage therebetween, said last mentioned means having an opening whichis positioned below the top surface of the upper reaches of said beltswhereby said sheets are prevented from adhering to the said openings,the second conveyor comprising a plurality of means each adapted tosupport a sheet thereon and each of said support means being, sopositioned that they pass through the plane of the belts of the firstconveyor without contact and to deposit each sheet on the belts of saidfirst conveyor.

11. In a sheet conveying apparatus the combination of a first and secondconveyor means the respective paths of which intersect at planessubstantially normal to each other, said first conveyor comprising atleast a pair of endless belt members, means to cause a low velocity andhigh volume of air to continuously pass downwardly only through thespace between said belts, said last mentioned means having an openingwhich is positioned in a plane below the top surface of the upperreaches of said belts, a frame pivoted at one end for supporting saidconveyor belts and said air means, means operatively secured to theotherend of said frame to retain the same in a substantially horizontalposition, means to release said frame retaining means, and meansresponsive to a downward pressure on said conveyor in excess of theweight of a sheet to actuate said frame releasing means.

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